1. Field of the Invention
The present invention relates to protector modules which are used in various locations such as telephone central offices to protect the inside equipment from damage as a result of overvoltage and overcurrent conditions that may occur on the incoming lines and more particularly to such a module which uses a gas tube and solid state devices such as diodes to provide overvoltage protection in combination with other solid state devices such as positive temperature coefficient resistors (PTCRs) to provide overcurrent protection.
2. Description of the Prior Art
There are many well known modules which are used at telephone company central offices and increasingly at other locations to provide protection against any overvoltage and overcurrent conditions that may occur on the telephone line pair. One example of such a module is that shown in U.S. Pat. No. 3,975,664 (hereinafter the '664 patent) which is assigned to the same assignee as is the present invention.
The module described in the '664 patent uses a three element cold cathode gas tube to provide protection against an overvoltage condition on the telephone line pair to which the module is connected. Protection against a sustained overcurrent condition on the line pair is provided by having slugs of low melting point solder in contact with the end terminals of the gas tube. The occurrence of a sustained overcurrent condition on either line of the line pair protected by the module causes the associated slug to melt. The melting of the slug allows the associated end terminal of the gas tube to come in contact with the ground terminal of the module. A more detailed description of how the module shown in the '664 patent provides overvoltage and sustained overcurrent may be obtained by referring to that patent.
The module described in the '664 patent does not provide protection against a marginal overcurrent condition. Therefore, it should not be used with those line pairs where such a condition may occur.
As discussed in the '664 patent, the module disclosed therein has certain advantages. Among them are the elimination of a spring to carry large overcurrents of long duration, relatively short current paths, standard 5 pin base, ease of assembly and its relatively small and compact configuration. It is desirable that such a module also be capable of protecting against marginal overcurrents.
The '492 application describes a protector module of the type disclosed in the '664 patent which uses solid state devices such as PTCRs to provide protection against marginal overcurrents. The use of such devices is desirable in that upon the occurrence of the overcurrent condition, the device does not permanently ground the telephone line. Rather the resistance of the device increases to limit the current. Upon the cessation of the overcurrent condition, the resistance of the PTCR returns to its ambient temperature value and the device is then ready for the occurrence of the next overcurrent condition.
It is also desirable that a protector module of the type disclosed in the '492 application include, in addition to gas tube protector element, solid state devices such as diodes which also function to protect against overvoltages. Typically, such solid state devices have a lower voltage rating than the gas tube protector element. While it is possible to design gas tube protector elements which have such lower voltage ratings this means that the gas tube may operate more frequently than if it has a higher rating. The more times that a gas tube operates, the more likely it is that the performance characteristics of the tube may be degraded. Protector modules which include both gas tube protector elements and solid state devices for overvoltage protection are described in U.S. Pat. Nos. 4,262,317 and 4,729,055. The '317 patent is assigned to the same assignee as is the present invention.